The present invention relates generally to rotating electrical machines, and more particularly, to a method and circuit for reducing ripple current in a multiple inverter system.
Multiple electrical machines or a single electrical machine having two windings or more that are each connected to a separate inverter circuit are typically operated in a fixed manner. Oftentimes, the switching of the inverter circuits occurs simultaneously resulting in a high ripple current in the DC bus. The result of a high ripple current is that a large capacitor must be used to help reduce the ripple current. In automotive applications, the DC bus is often connected to a battery which is sensitive to ripple current. That is, if a high ripple current is present on the DC bus the life cycle of the battery may be reduced. Also, the durability of the capacitor coupled to the DC bus is also reduced by a high ripple current, forcing the use of a higher ripple current rated capacitor which increases its cost. A high ripple current also increases the temperature of the components on the DC bus and increases the amount of electromagnetic interference generated by the circuit.
Particularly in automotive applications in which the numbers of units produced is extremely high, it is desirable to reduce the costs of components. The capacitors used to reduce ripple current in drive circuits are expensive components. The expense increases as the size of the capacitor increases. Therefore, it would be desirable to reduce ripple current and therefore reduce the size of the capacitor to decrease the cost of the circuit.
Known systems for reducing ripple current include operating the first inverter and a second inverter to cancel harmonics. Oftentimes the systems are operated with transformers that require a minimum inverter frequency at a maximum voltage. Therefore, one example of a phase displaced multiple inverter bridge circuit with waveform notching is described in U.S. Pat. No. 5,168,473. However, such a system is operated with a fixed phase difference between the inverters.
Because operating conditions, particularly in an automotive application, are continually varying, it would also be desirable to provide a system that varies the phase difference between the switching of the inverter circuits.
It is therefore one object of the invention to provide a circuit for controlling an electrical machine that can vary the phase between the switching of the inverters in response to the varying operating conditions of the circuit.
In one aspect of the invention, a circuit for controlling an electrical machine has a first set of windings with a first inverter circuit coupled thereto and a second set of windings having a second inverter circuit coupled thereto. A pulse width modulation controller pulse width modulates the first inverter circuit and the second inverter circuit so that the first inverter circuit has a predetermined phase shift therebetween to reduce output ripple.
One feature of the invention is that the pulse width modulating controller varies the phase difference between the first inverter circuit and the second inverter circuit in response to an operating condition of the electrical machine.
In a further aspect of the invention, a method for operating an electrical machine comprises the steps of pulse width modulating a first inverter circuit to have a first electrical phase angle;
pulse width modulating a second inverter circuit to have a second electrical phase angle;
sensing an operating condition of the electrical machine;
controlling the steps of pulse width modulating a first inverter circuit and pulse width modulating a second inverter circuit to reduce a ripple current in response to the predetermined operating condition.
One advantage of the invention is that the teachings of the present invention may be applied to control circuits for electrical machines that have more than two sets of windings and inverters. That is, three or more sets of windings and inverters may be simultaneously controlled by a pulse width modulating controller to reduce ripple current in the DC bus capacitors and associated components.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.